Supervisory system for communication circuits



April 16, 1946.

W. S. HALSTEAD Search Room SUPERVISORY SYSTEM FOR COMMUNICATION CIRCUITS Filed May 3, 1945 FIG.I

2 Sheets-Sheet 1 TALK INDIcAToR 2 LIGI-IT TRANSMITTER I .mrMlCROPHONE i I I PRESS-TO-TALK SWITCH H E-I4 --A.F. sIGNAL GENERATOR 46 QLDYNAMIC MICROPHONE TALK INDICATOR LIGHT PR ESS-TO-TALK SWITCH I---A.F. sIGNAL G NERATOR WILLIAM s I-IALsTEAb INVENTOR ATTORNEY ll-LL-I nuns,

Patented Apr. 16, 1946 Search Room SUPERVISORY SYSTEM FOR COMIVIUNICATION CIRCUITS William S. Halstead, Purchase, N. Y., assignor, by mesne assignments, to Farnsworth Television & Radio Corporation, Fort Wayne, Ind., a corporation of Delaware Application May 3, 1945, Serial No. 591,741

9 Claims.

This invention relates in general to radio communicating systems and is more particularly concerned with a radio communication system in which a constant supervisory check on the operation of central or other station transmitting equipment and cooperating mobile or secondary receivers is provided, for safety or for other operational reasons.

The system embodies and includes automatically operative devices whereby a checking" signal of frequency Fl, is normally emitted by a central station transmitter in the intervals between voice transmission periods. When the checking system is used to provide voice transmission, the frequency of the checking signal is shifted automatically to a second and different frequency, F2. which latter serves as a talk indicator" signal. The shift in frequency is automatically perceived by mobile or secondary station receiving equipment and is utilized to cause a normally-on check light at such receiving station to be extinguished and a "talk indicator" light alternatively to be excited, thus advising the operator of the mobile receiving unit that the control circuits at the central or transmitting station have been switched from their normal checking" interconnection to that employed during voice transmitting periods. Thus, by means of the system of this invention, an operational check of the positive two-signal type is provided, a first signal being used as a check upon the maintenance of radio contact between the control transmitting point and the mobile or secondary station receivers during the intervals between voice transmissions, and a second signal having some characteristic distinct from that of the first signal being used to indicate in a definite manner that the radio equipment at the control point has been placed in the voice transmitting position. This feature is of particular value when used in connection with mobile communicating systems in which the receiving vehicle may temporarily be in a region of low signal intensity, where the signal strength received from the transmitting station is not sufficient to operate the "squelch circuit of the receiver but where the signal intensity would be adequate for communication if the operator of the mobile unit employed the squelch cut-out" switch of the type normally used in vehicle radio equipment, such squelch circuits being well known in the art.

The system is also of particular value in selective calling systems employed in vehicular or other mobile radio services to indicate to the operators of receiving units not under call that the equipment is functioning properly during the central station's voice transmitting period, when the normal low frequency checking signal is not emitted. Under such conditions, in the case of systems constructed according to the prior art, there would be no energization of the check light at receivers not directly engaged in communication with the central station. Operators of receivers in vehicles not called would, in the case of the instant system, because of energization of the talk signal light, be advised in a positive manner of the fact that the central station was engaged in talking with another mobile receiving unit, this method of operation thereby providing a supplementary check for maintenance of radio contact at all times between the central station and all vehicles having receivers and situated within the eflective communications range of the central station.

One object of this invention is to provide a radio communication system in which a receiving station maintains constant supervisory check upon the operability of a transmitting station, whether such transmitting station be actually engaged in the transmission of speech or merely be in a stand-by connection.

Another object of this invention is to provide a communication system in which a plurality of receivers, designed to receive signals from a. common transmitter, are each provided with constantly operated supervisory signals, such as indicating lamps, such supervisory signals indicating to each receiver whether the transmitter is standing by or is emitting speech modulated energy, whether such speech modulation be intended for reception by a single receiver or by a number of such receivers.

A further object of this invention is to provide a radio transmitter in which the carrier is continuously modulated by an audio frequency current used for supervisory purposes, and in which such audio frequency current is caused to change in frequency whenever the transmitter is employed to send an intelligence-bearing message.

Yet another object of this invention is to provide a radio transmitter in which the emission of two alternate audio frequency checking signals serves to indicate to a receiving station, not only whether the transmitter is about to be used for message conveying, but also to indicate that the microphone circuit is in proper operative condition.

A still further object of this invention is to provide a radio communication system in which the transmitter, during. quiescent periods, 1. e. when no messages are being transmitted. emits a constant checking signal comprising a carrier modulated at a first predetermined supervisory audio frequency and, when a microphone or other message transmitting device is cut into the circuit as a step preliminary to message transmission, will continue to emit the same carrier, but will have impressed upon this carrier an audio frequency supervisory modulation of a second and different frequency, while the receiver is arranged to separate the two supervisory audio frequencies and the intelligence bearing frequencies from one another, so that the first supervisory frequency will afford a constant check upon the operability of the transmitter, while the second supervisory frequency will afford an indication that the transmitter circuits have been shifted so that they are then ready to transmit intelligence-bearing modulations.

Still another object of this invention is to provide a radio receiver for use in mobile units in cases where it is highly desirable that constant supervision of the integrity of the communication link between transmitter and receiver be maintained, such receiver being provided with supervisory signals, one of which is at all times functioning.

Another purpose of this invention is to provide a radio receiver in which the audio frequency currents derived from a signal demodulator are separated by frequenc discriminators into a number of paths, one path supplying a supervisory signal indicating that a transmitter is standing by, a second path supplying another supervisory signal indicating that a transmitter has been made ready to transmit a message, and a third path supplying intelligence-bearing signals to a loud speaker or other transducing device.

A further purpose of this invention is to provide a communication system from a single transmitter to a plurality of receiving units, for example from a. railroad yard main transmitter to a number of receivers each located in a. separate locomotive cab, in which the operator in each locomotive cab is provided with a visual checking signal indicating at all times whether the communication link is functioning properly, including the proper functioning of the transmitter and the receiver, while a different visual signal indicates to such operator that the transmitter is functioning, and in addition, that such transmitter is ready to send a message.

Yet another purpose of this invention is to provide a radio receiver adapted for the reception of a carrier which is alternatively modulated by two different audio frequency supervisory currents, and in which speech modulated currents can be received simultaneously with the reception of one of these supervisory currents, filtering or frequency discriminatory devices acting to separate such supervisory currents from the speech-modulated currents, so that a loudspeaker will be actuated solely by the speech-modulated currents while the supervisory currents serve to actuate a number of separate supervisory signals.

A still further purpose of this invention is to provide, in communication systems of the class just described, a receiver which will deliver to a loudspeaker substantially all speech modulated signals received thereby, while, at the same time, a plurality of supervisory modulating signals accompanying the speech signals are equalized in intensity by passing through a current limiting device, are then separated from one another by frequency discriminating devices and fina ly.

after suitable rectification, are employed to actuate a plurality of supervisory checking signals.

Still another purpose of this invention is to provide, in a communication system of the type above described, a transmitter of the type employing a press-to-talk switch, and also including a modulating tone generator capable of modulating the radio transmitter proper at two different audio frequencies and a relay switching device acting to alter the tone generator from one frequency to another, whenever the pressto-talk switch is depressed. Optionally, such switching relay may act effectively to connect a microphone into a modulating circuit, and also may determine the excitation of a supervisory indicator lamp, also located at such transmitter.

Still further purposes of this invention will be apparent from the following specification and the hereunto appended drawings.

The system of this invention may be considered somewhat analogous to frequency modulation systems, in that a number of different audio frequencies are used for transmitter modulation, and these frequencies are separated at the receiver by means of suitable frequency discriminators, so that each frequency perform a distinct function.

In the drawings:

Fig. 1 is a diagrammatic showing of one form of signal transmitter according to this invention.

Fig, 2 shows an alternative form of signal transmitter.

Fig. 3 shows one form of receiver for use with the transmitters of Fig. l or Fig. 2.

Fig. 4 shows an alternative form of receiver.

Fig. l is a diagram of one form of frequency shift checking system which can be employed in the present invention, as used in combination with a radio transmitter proper. As shown in the drawings, the radio frequency output of transmitter I2 feed antenna H. The transmitter input circuit consists of microphone It, input transformer l1, relay l9, press-to-talk" switch 20, talk indicator light 22, local battery 23 and a two-tone audio frequency signal generator 24.

An audio-frequency oscillator 24, of vacuum tube type, is employed in the illustrative embodiment of the invention as a source of two discrete audio frequency signals, utilized respec-. tively as checking" and "talk indicator control signals, the frequency of th audio oscillator being shifted by switch 20 and relay I 9 from a frequency such as Fl, which is designated as the checking signal, to a different frequency F2, which is designated as the talk indicator" signal.

The audio frequency signal generator 24 consists, in the present instance, of a pentagrid tube 26 and associated oscillatory circuit of wellknown ty having the audio output thereof feeding th primary winding of output transformer 21. Tuning of the audio oscillator to the desired audio frequencies is obtained in the audio frequency signal generator 24 by choosing the proper values for the inductance 32 and capacitor 33 and 34. Capacitor 38, resistors 81 and 38 and coupling capacitor 39 are also part of the signal generator and function as well known in the art. I' pad attenuator 28 is inserted in the output circuit of transformer 21 and is then connected to terminal 30 of the audio input circuit of the transmitter l2, so that the audio signal energy from the audio frequency signal generator is continuouslyapplied to the transmitter l2. Adjustable contact arm 28' of T pad attenuator 26 is utilized to maintain the required signal level output from the audio frequency signal generator 24.

When th press-to-talk switch 20 i not in use, as is the case in normal non-talking periods between voice transmissions, capacitor 33, alone, of the audio frequency signal generator 24 is connected across inductance 32, causing the checking signal to have a predetermined audio frequency, such as a 100-cycle tone. Thus, as the transmitter i2 i normally in continuou operation between voice transmission periods, the transmitter is modulated by the above-mentioned 100-cycle checking signals, thereby providing an audio-frequency checking signal Fl for the receiving equipment of the system, which will be later described. Separate 100 cycle and 40 cycle oscillators could be employed instead of a single oscillator arranged to generate two frequencies, as shown.

When the press-to-talk switch 20 is pressed, for initiating voice transmitting periods, the circuit from local battery 23 is completed and relay IQ is energized. When relay I9 is energized, normally-open contacts 4| are closed, thereby adding capacitor 34 in shunt across condenser 33, thus lowering or shifting the audio frequency generated by the audio oscillator from the nor mal 100-cycle frequency to a second, or talk signal frequency, such as 40 cycles. This 40- cycle frequency is below the normal telephonic voice frequency range and therefore will not interfere with voice signals. It is applied to the audio input circuit of transmitter I 2 in the same manner as the 100-cycle checking frequency previously described.

The energizing of relay l9 also close contacts 43, which latter connect local battery 23 to m crophone l6 and to the talk indicator light 22, allow ng the microphone to be used, and providing an indicating light, which shows that the micro hone is in operation.

Thus, in the pressto-talk position, the transmitter I2 is modulated by the 40-cycle talk si nal frequency, concurrently with modulation by speech si nals from the microphone I6. The 40-cycle frequency is then employed in receiving eclui ment, fed from the above described transmitter. for checking purposes, and to provide vi ual talk-signal indications, as will later be described.

Fig. 2 is a diagram of a modified form of the two-tone or frequency-shift checking ystem emnloyed in combination with a radio transmitter, in which a dynamic microphone 46 has been substituted for the battery actuated microphone described in Fig. 1.

The microphone circuit connects from input winding H of speech input transformer 1, throu h moving coil 46' of microphone 46 tothe T pad attenuator 28 inserted in the output circuit of transformer 21, and thence to a common ground. Adjustable contact arm 28' of T pad attenuator 28 is utilized to maintain the required signal level output from the audio frequency signal generator 24.

Provision is also made to prevent micro hone pickup during the transmission eriod of the check signal, by the following method.

Normally, as is shown in Fig. 2. the relay I9, when deenergized, causes the relay switch contacts 42 to maintain a direct short circuit across moving coil 46' of microphone 46 and by this means render the microphone 46 completely in-- operative and insensitive to ambient sound waves.

Search Room The microphone is restored to the transmitter input circuit by the closure of press-to-talk switch 20, causing relay l9 to operate, and the consequent separation of relay switch contacts 42 restores the moving coil 46 to the circuit of input transformer l1, thus rendering the microphone 46 ready for speech modulation. The remaining elements of the circuit are substantially the same as shown and described in Fig. 1, with the exception that the battery 23 of Fig. 2 does not here supply any current to the dynamic microphone 46, as it does in the form shown in Fig. 1. By the above described series circuit connection of the microphone and output circuit of the two-frequency audio frequency signal generator 24, a continual check is maintained on the audio input circuit, by the normally-transmitted 100- cycle checking signal, between voice transmitting periods and by the change in audio frequency from 100 cycles to 40 cycles during talk periods as described in connection with Fig. l. The above mentioned features provide additional and desirable checks of the entire system.

Fig. 3 is a block diagram of a receiving unit for operation with the transmitter equipment described in Figs. 1 and 2. In Fig. 3, the antenna 5| connects to the input circuit of receiver 52, such as a receiver on a vehicle, tuned to the fre-- quency employed by the transmitters [2, shown in Figs. 1 and 2. The output of receiver '52 feeds output coupling transformer 54, where the audio frequency components of the received signal are applied to band-pass filters, or frequency discriminators 56, 51, and 58, arranged as shown. Bandpass filter 56 will effectively pass voice frequencies in the band from 200 to 2000 or 3000 cycles or as much higher as desired, and has its output feeding loudspeaker 60, for the reception of incom ing speech signals. If it is desired also to re produce the 100-cycle checking signal from loudspeaker 60, the filter 56 is then designed to pass a band of audio frequencies extending from 100 cycles upward. Bandpass filter 51 is designed so that it will pass effectively only the 100-cycle checking signal Fl, normally emitted by the transmitter between voice transmitting periods. The output of band-pass filter 51 i then fed to rectifier 62, of any suitable type. The output voltage from rectifier 62 is applied to relay 63 which latter is energized when the -cycle checking tone is received, thereby closing normally-open relay contacts 64 and applying voltage from battery 66 to the check light 61. Band-pass filter 58 is designed so that it will effectively pass only the 40-cycle talk signal emitted during voice transmitting periods by the transmitter. Filter 56 has its output circuit connected to rectifier 69. The output voltage from rectifier 69 then energizes relay Ill when the 40 cycle talk signal is received, closing the normally-open relay contacts 72 and applying current from battery 56 to the talk signal light I4.

Fig. 4 is a. block diagram of a modified form of receiver somewhat similar to that indicated in Fig. 3, in which antenna 5| connects to the radio frequency input circuit of receiver 52. The output of receiver 52 feeds-output coupling transformer 54, which latter in turn feeds the band pass filter 56 and audio frequency amplitude limiter 55, of any suitable type. such as a saturated amplifier electronic tube, acting to prevent rise in signal voltage beyond a predetermined point of saturation, as well known in the art.

Band pass filter 56 passes voice frequencies in the 200-to-2000 cycle range and has its output circuit connected to loudspeaker 60 for speech reception. The output circuit of limiter 59 connects to band-pass filters 5'! and 58, as shown.

Band pass filters 51 and 58 and their corresponding rectifiers 62 and 69, in combination with relays 63 and operate in the same manner as described in connection with the arrangement of Fig. 3 except that the relays 63 and 10' are combined in this illustrative diagram in one unit, as is the case in some'forms of polarized relays having a common armature, so as alternatively.

to close contacts 64 or 12', as determined by the reception of the 100-cycle check tone or of the 40-cycle talk tone, as the case may be. Indicator lights 61 and 14, in combination with battery 86, also operate substantially as described in connection with Fig. 3.

The limiter 59 is likewise useful in preventing faulty operation of the relays, such as might be caused by appreciable amplitude changes in the received signals, or electrical noise, and renders the relays responsive only to frequency changes of the received tone signals which, by means of the discriminators or filters, 51 and 58, selectively control the relays 63' and 10', respectively.

The limiter is preferably adjusted so that it will be saturated whenever the receiver is positioned Within the zone of normal communications, thereby rendering more uniform the response of the checking relays and signals, and rendering the system as a whole relatively insensitive to false signal reception or to cross channel interference at the different tone frequencies.

The receiving arrangements described in Figs. 3 and 4 provide the operator with a continuous visual check on the operation of the entire transmitting and receiving system, by means of the check light 61, which latter will normally be energized during the intervals between voice transmitting periods, during which time the OU-cycle check frequency is impressed on the speech input circuit of the transmitter. When the operator at the transmitting equipment 0perates the press-to-talk switch 20, Figs. 1 and 2, the frequency of the audio frequency signal generator 24 is thereby shifted to 40 cycles, which latter modulates the transmitter, thereby causing emission of a carrier simultaneously bearing voice and talk signal frequencies. These signais, when received by the equipment described in Figs. 3 and 4, energize the talk signal light 14, and the loudspeaker 60, thereby providing visual, as well as audible indications to the operator at the receiving point that the transmitting equipment has been placed in the voice transmitting position. If the operator at the receiver hears no voice signal after the check signal light has been extinguished and the talk signal light has been energized, an indication of faulty operation of the over-all voice communications system is shown, such as a possible failure of the microphone circuit at the transmitting point. The operator at the receiver, which may be located in a locomotive or other vehicle, then knows that he cannot depend on the radio communications system for further instructions, and in the case of railroad installations of the equipment he may follow any suitable estabilshed precautionary procedure. If no signal whatsoever is received by the mobile receiver, an indication of failure of equipment is likewise given;

What is claimed as new and desired to secure by Letters Patent of the United States, is:

l. A speech signaling system of the type described, including a transmitter and at least one receiver, said transmitter including tone generator means producing a current having a first audio frequency under no-speech conditions, means for altering said frequency to a second value when speech transmission takes place, a carrier current generator, and means for alternatively modulating the produced carrier current by currents of both said audio frequencies and by speech signals and each of said receivers including means for receiving and demodulating said carrier current, filter means separating the modulations derived from said carrier current into three channels. the first channel delivering substantially only said current of first audio frequency and including first signal means operated by said delivered current, the second channel delivering substantially only said current of second audio frequency, and including second signal means operated by said delivered current, and the third channel delivering substantially only speech signal current free from both said audio frequency tone currents and including speech reproducing means operated by said delivered speech signal currents.

2. A frequency discriminating signaling system including a source of audio tone signals, a transmitter modulated by said tone signals, means for also modulating said transmitter by voice frequency currents, switching means for simultaneously changing the frequencyof said tone signals when said transmitter is modulated by said voice frequency currents, a receiver actuated by the output of said transmitter and including a demodulator, frequency discriminating means fed by said demodulator and separating said voice frequency currents and each of said tone signals of different frequency from one another, a loud speaker operated by said separated voice frequency currents, and signal transducer means actuated respectively by each separated frequency of said tone signals, whereby one of said signal transducer means indicates the periods when said transmitter is being modulated by said voice frequency currents and the other signal transducer means indicates the operative functioning of said transmitter when voice frequency modulation is not being employed thereat.

3. In a constant supervision signaling system, a transmitter including, tone generator means and a microphone, a carrier current producer connected so as to be modulated continuously by the output of said tone generator, and intermittently by the output of said microphone, switching means simultaneously connecting said microphone to said carrier current producer, altering the frequency of said tone generator means, and giving a, visual indication of said connection and said alteration, and including, a receiving station including, a receiver and a demodulator, a first frequency discriminator rejecting both tone frequencies and fed by said demodulator, a limiter also fed by said demodulator, and two other frequency discriminators each fed by said limiter and respectively passing substantially one only of said tone frequencies, and separate signal transducers, each fed by the respective output of one of said frequency discriminators, whereby the transducer fed by said first discriminator reproduces the speech impressed on said microphone and each other transducer indicates a, respective position of said switching means, whereby supervision of said transmitter is maintained both during microphone modulation thereof, and during stand-by periods.

4. A constant supervision carrier signaling system including, at the sender, a carrier generator.

Search Room a, tone generator connected so as continuously to modulate the carrier, a dynamic microphone connected between said two generators, means normally shunting and rendering said microphone inoperative, a switch simultaneously altering the frequency of said tone generator, removing said microphone shunt, and giving visual indication of said alterations, and including at the receiving station a receiver and a demodulator, a first frequency discriminator and a limiter, both fed by said demodulator, said discriminator rejecting both the tone frequencies but passing voice frequencies, two other frequency discriminators, both fed by said limiter and each passing a respective one only of said tone frequencies, and discrete transducers actuated by the respective outputs of said three frequency discriminators, whereby the transducer actuated by said first discriminator responds to the voice frequencies derived from said microphone and the transducers actuated by said other discriminators indicate respectively the position of said switch.

5. A signa1 transmitter giving constant indications of operability, including a carrier generator and a supervisory tone generator modulating the carrier, a microphone also connected to modulate said carrier, and a microphone actuating switch for simultaneously placing said microphone in operative condition and changing the frequency of said tone generator, whereby said transmitter radiates one supervisory tone frequency when quiescent and another supervisory tone frequency when microphone modulated.

6. A signal receiver adapted to be used with a transmitter of the type described, including a carrier receiver, a demodulator, a loud speaker, two supervisory signaling devices, and three frequency discriminators, all said discriminators being fed by said demodulator, the first discriminator passing voice frequencies minus tone frequencies and feeding said loud speaker, the second discriminator passing one tone frequency and feeding one of said supervisory signals, and the third passing another tone frequency and feeding the second of said supervisory signals.

7. A frequency discriminating signal system including a source of audio tone signals, a transmitter modulated by said tone signals, means for also modulating said transmitter by voice frequency currents, switching means for simultaneously changing the frequency of said tone signals and connecting said transmitter so as to be modulated by said voice frequency currents, a receiver actuated by the output of said transmitter, said receiver including a demodulator. a first frequency discriminator fed from said demodulator and filtering out said voice frequency currents, a

loudspeaker actuated thereby, a current limiter fed from said demodulator, two other frequency discriminators fed from said current limiter, each passing a respective one of said frequencies of said tone signal, and two rectifiers and two signal means connected thereto, each rectifier and corresponding signal means being fed by the respective output of one of said two last frequency discriminators, whereby supervision of the position of said switching means is obtained.

8. In a checking signal radio communications system, a transmitter including means for generating and radiating a carrier current, a microphone for modulating said carrier generating means, means for switching said microphone in and out of use, a generator of two alternative checking tone frequencies, switching means for connecting a first checking tone to said generating means during times when said microphone is not in use, and alternatively for connecting a second checking tone and said microphone simultaneously to said generating means, and a receiver including, means for receiving and demodulating said carrier current, frequency discriminatory means for substantially separating both said tone frequencies from the microphone modulated frequencies and from one another, and a plurality of transducer means respectively actuated by the output of each of said frequency discriminatory means, so as to afford separate indications for each tone frequency and for the microphone modulated frequencies.

9. A voice communication system of the constant supervision type including, at the transmitter, means for producing a carrier, means for modulating said carrier at a first audio frequency between voice modulation periods, means for alternatively modulating said carrier at a second audio frequency together with voice frequencies, an indicating light, and means for automatically actuating said light when modulating at said voice frequencies, and including, at the receiver, means for receiving and demodulating said carrier, two filters fed from said demodulating means, each passing a predetermined one of said audio frequencies, a third filter fed from said demodulating means for rejecting both said audio frequencies and passing the voice frequencies not including said audio frequencies, and discrete signal transducing devices actuated respectively by the respective outputs of said three filters, whereby one signal transducing device indicates the standby operation of the transmitter and the two other signal transducing devices respectively indicate the voice modulation of the transmitter and reproduce the transmitted voice frequencies.

S. HALSTEAD. 

